Fine perforations have been used successfully as a security feature for security documents, i.e. for documents the authenticity of which can be verified in a reasonably reliable manner, such as bank notes, passports or parts thereof, checks, etc.
WO 97/18092 describes a security document having a pattern of fine perforations that are visible when viewed in transmission while they are invisible when viewed in reflection. Even though this feature has found to be a very reliable means for authenticating the document, it is desired to increase the uniqueness of these and similar perforations in order to provide an even higher degree of recognizability and reliability.
WO 00/43216 teaches, inter alia, to add perforations that extend obliquely through the document and that must be viewed under a given angle. However, manufacturing such oblique perforations is difficult and their quality is likely to degrade over time, in particular when used for paper or thin plastic sheet documents that are subjected to frequent mechanical stress, such as bank notes.